Reported practice · r/Kintsugi

Drilling pin holes and sealing clay crack faces before mugi-urushi assembly

How to prepare drilled pin holes and the surrounding clay surfaces so that a mugi-urushi joint will bond reliably in a kintsugi repair of a ceramic mug, and whether the pins should be long enough to crimp inside the holes.

This page is written from one discussion thread. It records what practitioners reported and what they linked it to — not documented workshop practice. Every claim below links to the comment it rests on, so you can read it in context.

Safety

Ki-urushi (raw lacquer) is a potent skin sensitiser; contact can trigger a delayed urushiol-type rash that may appear 24–72 hours after exposure. Wear nitrile or latex gloves and avoid bare-skin contact with the liquid and with any surface it has touched.

What the thread establishes

The answer.

The practitioner drills pin holes with a diamond burr on a rotary tool, sizing them 2–3 mm deep and slightly wider than the 1 mm steel wire to allow a small amount of play. After test-fitting every joint to confirm alignment, all surfaces and the interior of each hole are flushed with soapy water through a syringe fitted with a blunt needle, then repeated with plain water, to remove every trace of drilling dust that would otherwise prevent the mugi-urushi from adhering. Once the pieces are clean and dry, a thin coat of ki-urushi is dabbed onto each crack face with cotton swabs to seal the porous clay body and stop it from wicking lacquer out of the mugi-urushi during assembly. The sealed faces are then left to cure for roughly a day before the pinned mugi-urushi joint is made. For the pins themselves, the practitioner adds small light pinches around the surface with wire clippers to increase the area available for the urushi to grip.

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What goes wrongResidual drilling dust left inside the holes or on the crack faces will inhibit the mugi-urushi from adhering properly to the clay. Separately, an unsealed porous clay body will absorb lacquer out of the mugi-urushi during assembly, weakening the bond.

What recovers itFlush the holes and surfaces with soapy water via a syringe, rinse with plain water, and allow to dry completely before proceeding. Apply a thin ki-urushi seal coat to the crack faces and let it cure (about a day) before applying the mugi-urushi joint.

Drill and size pin holes

Use a diamond burr on a rotary tool to drill holes approximately 2–3 mm deep, made slightly wider than the 1 mm steel wire so the pin has a small amount of play. Test-fit every pin in its joint to confirm that each hole aligns with its counterpart on the mating piece.

the comment this came from ↗

Flush out all drilling dust

Squirt soapy water into each hole with a syringe fitted with a blunt needle small enough to fit inside, then repeat the flush with plain water. Wipe the surrounding surfaces clean. The goal is to remove every particle of powder residue, because any remaining dust will prevent the mugi-urushi from bonding to the clay.

the comment this came from ↗

Seal the crack faces with ki-urushi

Once the pieces are clean and fully dry, dab a thin layer of ki-urushi onto each crack face using cotton swabs. This seals the porous clay body so it does not wick lacquer out of the mugi-urushi during the subsequent assembly step.

the comment this came from ↗

Cure the seal coat

Allow the ki-urushi seal to cure for approximately one day before beginning the mugi-urushi pinned assembly.

the comment this came from ↗

Texturise the pin surface

Use wire clippers to make a series of small, light pinches around the circumference of each pin. The added surface texture gives the urushi more area to grip, improving the mechanical bond between pin and joint.

the comment this came from ↗

··Reported

What was reported

Drilling dust left in pin holes or on crack faces will prevent mugi-urushi from adhering properly to the clay body. ↗
A thin ki-urushi coat dabbed onto the crack faces before assembly prevents the porous clay from absorbing lacquer out of the mugi-urushi joint. ↗
Adding small pinched textures to the pin surface with wire clippers increases the area available for urushi to grip, improving the bond. ↗
Pin holes are sized slightly wider than the pin to allow a small amount of play, and are drilled to a depth of roughly 2–3 mm for a 1 mm wire. ↗

≠Disputed

Whether pins should be longer than the drilled holes so they crimp inside the joint

The thread does not settle this

Pins should extend beyond the hole depth so that, once the pieces are assembled, the excess length crimps and locks the joint mechanically. ↗
Crimping only makes sense with very thin, easily bendable pins. With the thicker steel wire used here, forcing the pin to crimp would crack the clay body before any deformation of the pin occurs, so the pins are kept to the hole depth. ↗

Limits of this source

What the thread does not answer.

The thread does not specify the exact mugi-urushi mix ratio (wheat flour to lacquer) or the humidity/temperature conditions for the one-day cure of the ki-urushi seal coat.
Whether the pinned mugi-urushi joint itself is subsequently finished in gold (the kintsugi signature) or left as a plain lacquer repair is not addressed in this step.
The long-term durability of the crimped-pin approach versus the flush-pin approach in a functional soup mug subjected to thermal cycling is not discussed.

Read the documented methods →